docs: def _eager_expr_from_flat_schema(flat_schema):
      """
      :type flat_schema: dict
      """
      result = []
      for path, join_method in flat_schema.items():
          if join_method == JOINED:
              result.append(joinedload(path))
          elif join_method == SUBQUERY:
              result.append(subqueryload(path))
          else:
              raise ValueError('Bad join method `{}` in `{}`'
                               .format(join_method, path))
      return result